This invention relates to electrode substrates for electrochemical cells, particularly polymer electrolyte membrane fuel cells (PEMFC) and Phosphoric Acid Fuel Cells (PAFC), and processes for their production.
A fuel cell converts fuel, such as hydrogen, and an oxidant, typically oxygen, to electricity and reaction products. This electrochemical reaction is facilitated by electrocatalysts, typically from the platinum group.
Fuel cells typically are constituted of units, as shown in FIG. 1, called single cells 1, comprising an electrode assembly 1xe2x80x2 where a membrane or electrolyte layer 2 is sandwiched between two electrodes 3 and 4, individually referred to as anode 3 and cathode 4. These electrodes are typically flat and have at least two parallel surfaces, the membrane or electrolyte layer 2 being positioned between these surfaces of the two electrodes.
Each of the electrodes 3 and 4 is composed of a porous conductive electrode substrate 3xe2x80x2 and 4xe2x80x2, usually made of carbon fiber paper or carbon cloth, and a thin electrocatalyst layer 3xe2x80x3 and 4xe2x80x3, preferably comprising finely divided platinum or other noble metal catalysts.
When using hydrogen as fuel, the fuel gas is oxidised at the anode 3 yielding protons and electrons. The former migrate through the membrane layer 2 from the anode to the cathode 4, while the electrons are transported through an external circuit to the cathode 4. At the cathode 4, oxygen is reduced by consumption of two electrons per atom, to form oxide anions which enter the electrolyte layer and react with the protons that have crossed the electrolyte layer to form water. As shown in this FIG. 1, separator plates 5 and 6 which are adjacent to the electrodes 3 and 4, may incorporate grooves 8 and 9 on the surfaces opposite to the electrodes providing access for the fuel and oxidant to the electrodes. The separator plates 5 and 6 can be covered with current collector plates 7 and 7xe2x80x2 usually made of metal which also act as conductive connection between two adjacent single cells.
PEMFC generally employ a membrane electrode assembly (MEA, 1xe2x80x2) as single cell comprising a thin polymer membrane 2 with high proton conductivity placed between two electrode sheets 3 and 4. PAFC single cells are typically constituted of a thin phosphoric acid containing matrix layer 2 sandwiched between the two electrodes 3 and 4.
The electrodes 3 and 4 mainly comprise of an electrically conductive and chemically inert electrode substrate (ES) 3xe2x80x2 and 4xe2x80x2 and an electrocatalyst layer (3xe2x80x3 and 4xe2x80x3) facing the membrane or electrolyte 2. The ES has a porous structure to provide an efficient entry passage and planar distribution for the fuel and oxidant to the catalyst layers 3xe2x80x3 and 4xe2x80x3 as well as an exit for the reaction products away from the catalyst layer. It also features other important properties such as high electrical conductivity, chemical stability, mechanical strength, and homogeneity.
As is shown in FIG. 1, it is advantageous to separate the functions of providing access and distributing fuel and oxidant (established by the grooves 8 and 9 in the separator or distributor plates 5 and 6 in FIG. 1) and the support of the catalyst layer 3xe2x80x3 and 4xe2x80x3 by the electrode substrates 3xe2x80x2 and 4xe2x80x2. The separator or distributor plates 5 and 6 are usually made of metal or other conductive materials as they shall also serve to collect the current. They incorporate grooves 8 and 9 or other means of distribution of liquids or gases. These separator plates are stacked on the electrode substrates on the side opposite the electrolyte layer 2.
Current can be collected in the distributor or separator plates (as mentioned above), or in separate current collector plates which can be a solid metal sheet if they form the outer part of the assembly, or can be a mesh or porous conductive plate if they are stacked between the fuel feed and the electrodes (between 4 and 6, or between 3 and 5, in an assembly as otherwise shown in FIG. 1). It is also possible to combine the separator plates and current collector plates.
Since various gases and liquids have to permeate through the ES, high porosity is a preferable feature of an ES. At the same time, the pore size distribution needs to be adjusted to the general characteristics of practical fuel cells. The grooves in the electrode substrates provide a very coarse distribution of fuel and oxidant. These need to be evenly transported and finely distributed to the catalyst layer through the ES. Furthermore, various types of gases and liquids have to be transported through the ES which requires fine-tuning and adaptation of the ES porous network. Hence, adjusting the degree of porosity as well as pore size and its distribution of an ES is important for the performance of a fuel cell.
Equally important is the through-plane (perpendicular to the large surface) electrical conductivity of the ES since they provide a conductive path between the catalyst layer and the separator or current collector plates. A low electrical conductivity can result in substantial power losses of the fuel cell. Usually, high porosity of an ES has to be balanced against improved through-plane conductivity or vice versa.
Mechanical properties of ES play an increasingly important role for the production of commercial fuel cells since the ES are being handled by automatic equipment, and product integrity determines the commercial success of fuel cells.
In the light of fuel cell commercialisation efforts, ES are also required to be processable as a continuous roll material. This allows the application of industrial scale processes for the catalyst layer deposition and other required manufacturing steps.
Furthermore, a continuous roll ES provides high homogeneity and product uniformity in comparison with ES produced in a batch-mode.
Commonly used ES materials for fuel cells include carbon fibers (papers, felt, and woven cloth), metal fibers (mesh or gauze), and polymers (gauze filled with carbon materials).
A carbon fiber paper ES is usually made in such way that the carbon fibers are aligned mainly in planar direction. Due to the high anisotropy of carbon fibers, the in-plane conductivity of such carbon fiber paper is high but through-plane conductivity is poor. Such carbon fiber paper can be rendered suitable as ES for fuel cells if it is manufactured using a carbonisable binder followed by carbonising this product at high temperatures to achieve satisfactory through-plane conductivity (cf. U.S. Pat. No. 4,851,304). This type of ES is shown as a cross-section in FIG. 2. Carbon fibers 10 are aligned mainly in planar direction; carbonised binder particles 11 contribute to the mechanical stability of the ES. Carbonisable binder in this context means a binder, usually a binder resin which cross-links under the action of heat, that can be converted to elemental carbon in a high yield when heated for a prolonged time, i. e. more than 5 minutes up to several hours, above the decomposition temperature with the exclusion of oxygen or oxidising gases. This expensive batch-process yields ES with poor mechanical properties. WO 98/27606 relates to a process for filling carbon fiber papers and polymer substrates having low through-plane conductivity with conductive materials. The ES resulting from this procedure still lack a high through-plane conductivity and have a low porosity because the pores of the starting materials have to be filled with a high fraction of conductive material to achieve a sufficient level of through-plane conductivity.
Woven carbon cloth can be utilised as ES base material, but it is expensive and restricts the options to reduce the ES thickness. Metal fibers suitable for fuel cell ES are expensive since they need to be oxidation and corrosion resistant, and therefore must be selected from the noble metals such as platinum, iridium, rhodium, or osmium.
Consequently, what is required is a low-cost ES with high porosity as well as through-plane conductivity which is manufactured using an industrial scale continuous production process.
According to the present invention, electrode substrates for electrochemical cells, more specifically for fuel cells, with high porosity and good electrical conductivity and methods for their manufacture are disclosed. The electrode substrates comprise a carbonised or graphitised fiber (also often referred to as xe2x80x9cgraphite fiberxe2x80x9d) web structure with a high electric through-plane conductivity, said web structure being covered and filled with impregnation agent, and optionally, with chemically inert and conductive particles.
The ES described in this invention are made from conductive preformed web structures based on graphitised fibers that preferably have a through-plane conductivity of more than 1 S/cm, more preferably 6 S/cm or more, and especially preferred in excess of 6.4 S/cm. Through-plane conductivity is determined as described in WO 98/27606, which is herein incorporated by reference.
The ratio of through-plane conductivity to in-plane conductivity of the ES according to this invention is usually at least 0.25, preferably more than 0.42, and especially preferred more than 0.66. In-plane conductivity can be measured by a similar method, wherein two pairs of contact blocks are pressed on an ES material, and a current of 3 Ampere is applied between the two pairs of contact blocks. In-plane conductivity is then calculated from the voltage drop between the two pairs of blocks, the applied current, and the cross-section of the substrate and the distance between the two pairs of blocks.
The web is characterised by a high fraction of graphitised fibers being oriented not in planar direction. Graphitised fibers are highly anisotropic, thus their conductivity along the fiber axis is superior to the conductivity perpendicular to the fiber axis. Therefore, a high fraction of graphitised fibers with non-planar orientation in a web structure results in a high through-plane conductivity. Such web structures comprise, but are not limited to, woven cloth, needled felt, hydroentangled non-woven, and knitted fabric. High fraction in this context means at least 20 percent, preferably, at least 30%, and most preferred, more than 40% of all graphitised fibers. Such a web structure is shown in FIG. 3. The graphitised fibers 10 form a web which imparts the preferential orientation to the fibers.
The current method to manufacture such graphitised fiber based web structures is to use oxidised polyacrylonitrile (PAN) fibers followed by graphitisation in batch or continuous furnaces. The utilisation of carbon fibers for manufacturing such structures is prevented by the high stiffness of carbon fibers. Even forming such web structures from oxidised PAN fibers results in low manufacturing speed and relatively high scrap rates because these fibers are also difficult to process because of their mechanical properties.
A method to circumvent these problems is the highly efficient production of such web structures directly from PAN fibers, such as Dolanit(copyright) 12-based PAN fibers, which are then treated in a continuous oxidation furnace as described in U.S. Pat. Nos. 3,914,960 and 5,853,429, followed by a graphitisation step. This entire process is very cost effective and yields a uniform continuous material.
The web structures need to be processed further, in order to adjust their porous structure, bending stiffness, thickness and other desired final properties.
For this purpose, the web structure is impregnated th a liquid which may contain chemically inert and electrically conductive particles. Those impregnated conductive web structures are calendered to adjust the final thickness and the material homogeneity. During this step, the calendered material is heated and dried.
In another embodiment of this invention, the calendering step is followed by a final heat treatment. The conditions of this final heat treatment procedure are determined by the final ES properties. FIG. 4 is a cross-section of such an impregnated web structure according to this invention. The graphitised fibers 10 forming the web structure are mainly aligned perpendicular to the planar direction (the horizontal axis in this figure) and are enclosed by the impregnation agent 12 and optionally the chemically inert and electrically conductive particles. The pores 13 are still large and their size and shape are adjustable according to the requirements of the particular fuel cell electrode.
The process according to the invention yields a roll of low-cost ES with final properties superior to the prior art products. Such ES rolls can be used for subsequent fuel cell electrode manufacturing steps on industrial scale.
The foregoing and other features and advantages of the present invention will become more apparent from the following description.